In the field of dc circuits it is desirable to control the electrode currents of opposing electrodes along a channel. For example, electrode direct-current distribution in linear MHD generators exhibits a nonuniform pattern along the length of the generator channel. Such nonuniformities are particularly intense in diagonally connected generators and can impose high direct-current densities on individual electrodes, which may lead to accelerated erosion of the electrodes and thus reduce channel life. It is therefore of paramount importance to achieve direct-current control in dc circuits of the type having opposing electrodes along a channel, and more particularly in MHD generators which are diagonally connected.
In one approach, circuits to control diagonal currents in such diagonally connected generators have involved the use of resistive elements having nonlinear characteristics. Several such circuit arrangements are described in U.S. Pat. Nos. 3,940,639 (Enos et al) and 3,940,640 (Petty et al), which are incorporated by reference herein. As described in these patents, both active and passive impedances have been employed, and both the current flow between diagonally opposed electrodes and the voltage between adjacent discrete electrodes have been controlled. More specifically, the current control circuits have employed passive impedances such as direct current (dc) resistors and current limiter active circuits which produce a voltage drop sufficient to limit the current to each electrode to a predetermined value. One such current limiter circuit includes a conventional current sensor in the connecting line between diagonally opposed electrodes, the output of which is used to control the gain of a power transistor or like connected in series with the current sensor.
In one embodiment of the resistive voltage control circuits, a resistance is connected in series with a zener diode between adjacent electrodes. The voltage produced at the junction of the resistance and the zener diode controls a power transistor connected between the electrodes to oppose any tendency of the Hall voltage between the electrodes to exceed a limit determined by the resistance and diode. In a second embodiment of the voltage control circuits, a ballast resistor and power transistor switch and zener diode are connected in series between adjacent electrodes. The transistor switch is controlled by a sensing resistor connected between the electrodes.
Resistive controls, such as those described herein above, are dissipative, and hence, are not preferred for power producing MHD generators.
Another approach to controlling the performance of dc MHD generators is disclosed in U.S. Pat. No. 3,792,340 (Sheinkman et al), which is incorporated by reference herein. The Sheinkman et al approach involved utilizing the value of the MHD generator total power as a control criterion. More specifically, the voltages across the generator electrodes are periodically varied in a discrete manner. The change in the generator active power is determined for every time interval equal to the period of the discrete voltage variations, and control signals are generated in dependence on the changes in the active power so as to maintain a preset electrical load factor. In a preferred embodiment, the change in the active power is determined by converting the integral of the active power within the cycle of an alternating current (ac) main voltage into a power pulse proportioned to the integral and equal to the difference between two successive pulses constituting a pair. However, this type of approach is quite complex to implement, and moreover, does not attempt to control the plasma parameter variations between electrodes. It is also desirable to reduce axial currents along the generator channel.